Acoustic image conversion tube

ABSTRACT

3. An underwater acoustic device comprising in combination an annular support member, a vacuum backed circular piezoelectric plate mounted on said support member, a substantially mechanically rigid wall element mounted on said support member and forming with the front face of said plate an enclosure, and an acoustic energy propagating liquid filling the free space within said enclosure, said liquid at all times being under a pressure no greater than atmospheric but not less than the vapor pressure of said liquid at the temperature of the water in which the device is to be utilized.

agemann Mar. 26, 11974 [54] ACOUSTIC KMAGE CONVERSION TUBE 2,434,6661/1948 Mason 340/8 PC [75] Inventor: .llulius Hagemann, Panama City,Fla. [73] Assignee: The United States 01 America as l;

represented by the Secretary of the Navy, Washington, DC. 2983900 5/1961[22] Filed: Sept. 2, 1961) Primary Examiner-Richard A. Farley [2]] AppLNO: 53 850 Attorney, Agent, or FirmLouis A. Miller; Rolla N.

Carter [52] US. Cl. 340/5 MP, 315/11 340/10 EXEMPLARY CLAIM [51] lint.C1. 11104111 11/00, H01 31/495 58 mm 111 Search 343/17; 178/6.8, 5.6,5.8, mpmmg 178/6 SP 7.5 E 7.82 7.8' 313/148 1 bination an annularsupport member, a vacuum 7 backed Circular piezoelectric plate mountedon said 146 3 support member, a substantially mechanically rigid 315755i 1 1 b wall element mounted on said support member and /1 340/5 8 d 8 6forming with the front face of said plate an enclosure, and an acousticenergy propagating liquid fining the [56] References Cited free spacewithin said enclosure, said liquid at all times being under a pressureno greater than atmo- UNITED sTTEs PATENTS spheric but not less than thevapor pressure of said liquid at the temperature of the water in whichthe de- O O O 3,414,827 1/1947 Mason .1 340/10 me Is to be utlhzed'2,416,314 2/1947 Harrison 340/8 MM 3 Claims, 2 Drawing Figures ACOUSTICIMAGE CONVERSION TUBE The invention described herein may be manufacturedand used by or for the Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

The present invention relates to tubes of the cathode ray type forconverting an acoustic image into a video signal and more particularlyto such tubes constructed to be robust enough to withstand pressures ofseveral atmospheres without sacrificing desired operatingcharacteristics or performance.

A slab of piezoelectric material such as quartz or polarized bariumtitanate having lateral dimensions large compared to the wavelength ofthe acoustic energy present in an acoustic image applied to anelectroded side of the slab through a coupling medium will producelocalized electrical effects such that an electrical image will appearon the opposite bare side as an alternating potential pattern having thefrequency of the acoustic excitation. If this piezoelectric slab, whichwill hereinafter be referred to as the conversion plate, is uti lized asthe front face of a cathode ray tube with its bare face inward; electronbeam scanning of this bare face will detect the point-by-pointvariations in electric charge distribution thereon corresponding to theacoustic energy distribution on its outer face and generate a timevarying signal, i.e., a video signal, which by a system common totelevision practice can be reproduced as a visual image on the screen ofa cathode ray oscilloscope to display point-by-point the intensity ofthe acoustic energy coming from corresponding points in the object fieldimaged on the front face of the conversion plate. Such a system can notcompete with television in any medium transparent to light but inmediums opaque or substantially opaque to light such as turbid watersuch a system is useful and at the present time is the only one knownwhich will present to remote viewers a picture of an object submerged inturbid water.

The optimum thickness for the conversion plate is a half wavelength ofthe acoustic energy utilized to form the acoustic image and for bothquartz and barium titanate this corresponds to a thickness ofapproximately 0.1 inches at a frequency of 1.0 megacycle per second(mops). if a half wave conversion plate alone constitutes the end of theimage tube, the maximum diameter imposed at a pressure of one atmospherefor a frequency of 1.0 mcps is about 6.0 inches for quartz and about 4.4inches for barium titanate. Waters to a depth of 180 feet are usuallyassumed to be mineable and if the acoustic image tube is to be employedfor mine hunting at such depths it would be subjected to pressuresexceeding six atmospheres and obviously the diameter of an image tubeemploying only an unreinforced conversion plate is severely limited. Theapparent size advantage afforded by quartz is mainly illusory because ofits unavailability in sizes much greater than 3 inches except as museumpieces. Strength can be gained by dimensioning the conversion plate anodd multiple of one-half wavelength thick but in the present state ofthe art this introduces shortcomings which are unacceptable. it has beenproposed to solve this strength problem by bonding the conversion plateto a metal front plate which forms an integral part of the vacuum tubeenvelope. This sandwich" type construction is difficult to bond on themolecular-scale necessary and the metal plate constitutes a necessaryevil in the acoustic path.

In accordance with the present invention the image tube is constructedwith a pressure relief barrier between the water medium and theconversion plate which effectively protects the conversion plate withoutadversely affecting acoustic performance; and, indeed, may wellconstitute a welcome supplement to the acoustic imaging system.

An object of the invention is to provide an acoustic image conversiontube with a window structure which will withstand the pressuredifferential between the substantial vacuum of the tube and thehydrostatic pressure at substantial depths under water.

Another object of the invention is to provide a pressure relief barrierbetween the conversion plate of an acoustic image tube and thesurrounding medium.

The features of this invention which are believed to be novel are setforth with particularity in the appended claims. The invention itself,however, both as to its structure and operation together with furtherobjects and advantages thereof may best be understood by reference tothe following description taken in connection with the accompanyingdrawing in which:

FIG. 1 is a diagrammatic representation of an acoustic-to-visual systemin which the invention is utilized; and

FIG. 2 is a fragmentary view in section ofa relief barrier mounted overthe conversion plate of an acoustic image tube.

Referring now to the drawing, FIG. 1 represents an apparatus forclassifying and identifying objects under water by forming from highfrequency acoustic energy reflected by such object an acoustic image ona conversion plate of piezoelectric material, scanning the resultingelectrical image on the opposite side of the conversion plate to developa corresponding video signal and synthesizing a visual image from thisvideo signal. This apparatus as shown includes an acoustic lens 10 ofsuitable material such as methyl methacrylate plastic positioned toimage an object field on the conversion plate 11 of suitable materialsuch as quartz or barium titanate, forming the front end of a cathoderay tube 12 comprised of a generally cylindrical envelope 13 having anelectron gun l4 and a screen mesh electrode 15 positioned to collectsecondary electrons resulting from the scanning of the rear surface ofthe conversion plate ill. The collector electrode 15 is connectedthrough an external terminal 16 to a preamplifier 17 which preferably isincluded in the underwater assembly with the tube 12. The output of thepreamplifier I7 is fed through a video amplifier 18 to the intensitygrid 19 of a cathode ray tube 21 having on its face a fluorescent screen22. Each of the tubes 12 and 21 is provided with horizontal and verticaldeflecting coils 23 and 24, respectively, which are connected to beenergized in synchronism by suitable sweep circuits 25 in accordancewith common practice in the television art. Also in accordance withcommon practice, the vertical return trace in the cathode ray tube 21may be blanked through the video amplifier 18 under the control of asignal carried over a lead 26 from the vertical sweep generator in thesweep circuits 25.

Referring now to FIG. 2, wherein parts in common with FIG. I are givenlike reference characters, the conversion plate 1 1 is mounted on anaxially compliant metal bellows 30 which is highly resistant to radialcollapse. The bellows 30, which may be nickel, is fused to the open endof the glass envelope 13 of the camera tube I2. Adjacent this end of thetube 12, a circumferential flange 31 is formed, preferably integrallywith the envelope 13, to serve as a support for a cup shaped member 32having its concave surface facing the conversion plate 11 and preferablymade of an impedance matching material such as molded Rho-C rubber. Thecentral area of the cup 32 in axial alinement with the conversion plate11 is shown as having a meniscus shape of power, i.e., its inner andouter spherical surfaces are concentric. As was indicated above, thiscentral portion of the cup 32 may be other than 0 power to form anactive part of the imaging system. It will be understood that theacoustic thickness of this central portion of the cup 32 is preferablyan integral multiple of /2 wavelengths of the acoustic energy projectedtoward the object field being observed. The peripheral portion of thecup 32 is cylindrical and surrounded by a snug ring 33, preferablymetal, for restraining the cup 32 against radial expansion when theconvex surface of the cup 32 is subjected to hydrostatic pressure andinasmuch as the cup 32 is made of substantially incompressible material,it will be evident that only a very small deformation or axialdisplacement of the cup 32 can take place. The volume enclosed by thecup 32, the

conversion plate II, the bellows and the flange 3R is filled with acarefully degassed liquid 34, such as castor oil, having the properimpedance for transmitting the acoustic energy to the conversion plate11. Thus any small displacement of the concave surface of the cup 32will result merely in a slight compression of the bellows 30 withoutplacing any additional stress on the conversion plate 111 other than theforces resulting from the compression of the bellows 30 which will besmall because its compliance can be quite high and still avoid radialcollapse. Also, it is preferred to have the liquid 34 at subatmosphericpressure so that even at the maximum depth during use the pressuredifferential on the plate RI never exceeds one atomsphere and preferablyis considerably less. In the arrangement just described, the collectorelectrode 15 is shown as a wire mesh screen supported by the metalbellows 30 and connected to the external terminal 116 through a lead 35,the grounded electrode on the front surface of the plate 11 not beingshown.

In manufacturing an image tube of the invention, when the cup member 32and the liquid 34 are of materials which can withstand the outgassing ofthe tube 112, the pressure barrier assembly will be mounted on the tub12 prior to its being evacuated and thus the bellows 30 will be in itsneutral position at this time. Later, upon evacuation of the tube 12 tosay 10' millimeters of mercury or less, the plate lll moves to reducethe pressure in the liquid 34 to equal the strength of the bellows 31)under compression, the pressure of the near vacuum in the tube 112 beingignored. It is thus evident that the conversion plate llll is at no timesubjected to a pressure differential on its opposite faces equal to oneatmosphere and in practice is made considerably less than one atmosphereby suitable selection of bellows strength but greater than the vaporpressure of the liquid 34 which is chosen to have a relatively lowvalue.

This of course means that the area of the conversion plate 11 is nolonger limited by the requirement that it withstand one atmosphere ofpressure but rather has its maximum area determined by a considerablylower pressure differential, i.e., substantially the vapor pressure ofthe liquid employed.

When the techniques employed for fabricating and outgassing the cathoderay tube 12 would deleteriously affect both the cup member 32 and theliquid volume 34 comprising the pressure relief barrier of theinvention, after the tube 12 has been outgassed these components can beassembled by the technique indicated in broken lines in FIG. 2. Theglass flange 31 is originally formed with diametrically opposedpassageways 36 and 37 (shown sealed off in FIG. 2) and connectedrespectively through conduits 38 and 39 to a reservoir 40 of oil orother liquid under subatmospheric pressure and to a vacuum source 41. Itwill be evident that as the vacuum source 41 removes the air from thevolume enclosed by the cup 32 and the conversion plate 11, the liquidfrom the reservoir 40 will flow into and eventually completely floodthis volume with the liquid 34. When this volume is completely filledwith the liquid 34, as may be indicated by a liquid trap 39, thepassageways 36 and 37 in the glass flange 31 are sealed off as shown inFIG. 2. By sealing the passageway 37 first, the liquid 34 is under theknown pressure of the reservoir 40. This technique results in the liquid34 being confined at less than atmospheric pressure which for thereasons pointed out above is desirable, it being remembered that thesubatmospheric pressure chosen is preferably slightly greater than thevapor pressure of the liquid employed.

When the liquid 34 but not the cup member 32 would be deleteriouslyaffected by the outgassing of the tube 12, the pressure barrier of theinvention can be assembled so that the conversion plate 11 is neversubjected to a pressure differential greater than a selectedsubatmospheric pressure by evacuating the volume to be filled with theliquid 34 simultaneously with the evacuation of the tube 12 and thenafter the tube 12 has been outgassed, the oil or other liquid from thereservoir 40 can be admitted to the volume 34 as above described.

While for the purpose of disclosing the invention to enable thoseskilled in the art to practice the same a preferred embodiment has beendescribed in detail, it is to be understood that the invention is notlimited to the precise structure shown and that many obviousmodifications may be made therein without departing from the spirit ofthe invention, the scope of which is pointed out in the appended claims.

What is claimed is:

H. An underwater acoustic wave system of the type employing a cathoderay tube having for its front face a piezoelectric plate for convertingan acoustic image formed thereon into a corresponding electrical chargepattern, characterized in that said tube has an axially compliantcylindrical extension on which said conversion plate is mounted, asubstantially rigid front wall member of acoustically transparentmaterial is secured to said tube to form with said conversion plate andsaid extension an enclosure, and a liquid fills the free space withinsaid enclosure for propagating acoustic energy between said front wallmember and said conversion plate.

2. Apparatus as defined in claim 1 wherein the liquid filling saidenclosure is under subatmospheric pressure not less than the vaporpressure of the liquid at the temperature of the water in which theapparatus is to be utilized.

within said enclosure, said liquid at all times being under a pressureno greater than atmospheric but not less than the vapor pressure of saidliquid at the temperature of the water in which the device is to beutilized.

1. An underwater acoustic wave system of the type employing a cathoderay tube having for its front face a piezoelectric plate for convertingan acoustic image formed thereon into a corresponding electrical chargepattern, characterized in that said tube has an axially compliantcylindrical extension on which said conversion plate is mounted, asubstantially rigid front wall member of acoustically transparentmaterial is secured to said tube to form with said conversion plate andsaid extension an enclosure, and a liquid fills the free space withinsaid enclosure for propagating acoustic energy between said front wallmember and said conversion plate.
 1. An underwater acoustic wave systemof the type employing a cathode ray tube having for its front face apiezoelectric plate for converting an acoustic image formed thereon intoa corresponding electrical charge pattern, characterized in that saidtube has an axially compliant cylindrical extension on which saidconversion plate is mounted, a substantially rigid front wall member ofacoustically transparent material is secured to said tube to form withsaid conversion plate and said extension an enclosure, and a liquidfills the free space within said enclosure for propagating acousticenergy between said front wall member and said conversion plate. 2.Apparatus as defined in claim 1 wherein the liquid filling saidenclosure is under subatmospheric pressure not less than the vaporpressure of the liquid at the temperature of the water in which theapparatus is to be utilized.